Stabilized grain size refractory metal powder metallurgy mill products

ABSTRACT

A powder metal (P/M) mill product and the method of fabrication such product made out of low oxygen (&lt;400 ppm) refractory metal, or alloys thereof, using oxide additive (such as MgO, SiO 2 , and Y 2 O 3 ) for co-fabrication to achieve refractory metal grain size stabilization as required in high temperature applications. One such product is a sheet with small grain size containing oxide particles as grain size stabilizers. The product has good mechanical properties, low oxygen content in refractory metal fiber derivatives of the powder within the mill product and if is available as large pieces of sheet (lateral dimensions). The metal powder is consolidated to a sheet bar by different methods, which may weigh 50 pounds or more.

BACKGROUND OF THE INVENTION

The invention relates generally to metal mill products (and fabricatedparts) made from powders of refractory metals including the elementalmetals and their alloys and, more particularly to the use of oxidedopants for grain size stabilization in mill products and fabricatedparts to be subjected to high temperature application usage and/or hightemperature fabrication processes.

Users of refractory metals have had a long-standing interest inreplacing tantalum with niobium. One driving force for such replacementof tantalum is price as well as the limited availability of tantalum.Many mill products involve high temperature exposure in fabricationand/or use. The high temperatures can cause grain growth. In variousapplications large grains, as a consequence of such grain growth, aredetrimental to the performance of the material. This has been alimitation of niobium substitution for tantalum. Other limitationsinclude lesser strength and hardness as-fabricated niobium and itsalloys.

Currently, areas of interest include furnace parts, sintering trays anddeep drawn cups as used for manufacturing synthetic diamonds. Theseproducts require material with small grain size. Furnace partsparticularly require the material to have slow grain growth duringservice in order to prevent premature deterioration of the mechanicalproperties.

Currently tantalum material with stabilized grain size, due to alloyingadditions or other artifacts, is used for wire or sheet. In oneembodiment or state of interaction, SiO₂ is used as a grain stabilizer.The disadvantage of such a manufacturing method (resistance-sintering)for grain size stabilized tantalum powder metallurgy (P/M) material isthat it is limited to a lot size of 30 pounds for tantalum andapproximately 15 pounds for niobium. It is desirable to make lot sizesof up to 1000 pounds of tantalum and 500 pounds of niobium respectively.

Current manufacturing methods for large P/M sheet sizes/strip length arenot capable of providing large pieces of sheet or long coils of sheetwith the same low level of oxygen content and good mechanicalproperties.

It is an object of this invention to provide a powder metallurgy (P/M)route to fabrication of refractory metals in large lots with low oxygencontent and to provide resultant mill products with low oxygen content.

It is a further object of this invention to provide a P/M source formill products and eventual mill products with a finer grain and adecreased grain growth than are achieved with ingot source materials.

These objects are applicable to refractory metals generally and moreparticularly to niobium and its alloys.

The objects set forth above as well as further and other objects andadvantages of the present invention are achieved by the invention asdescribed hereinbelow.

SUMMARY OF THE INVENTION

The invention relates to a process for making a metal mill product froma refractory metal powder comprising (a) providing a low oxygenrefractory metal powder; (b) adding to the powder a grain growthinhibitor to the low oxygen refractory metal powder before consolidatingthe powder, (c) consolidating the powder by either hot isostaticpressing, extrusion or another thermomechanical working process; and (d)subjecting the consolidated powder to subsequent thermomechanicalprocessing, and thereby forming the mill product. The invention alsorelates to products made from such a process.

Grain growth inhibitors are added to niobium powder by blendinginhibitors such as SiO₂ and Y₂O₃ prior to consolidation or as a residueof a de-oxidation process where magnesium is added to capture the oxygenfrom the niobium powder and form magnesium oxide during the de-oxidationprocess.

The powder is consolidated either by hot isostatic pressing (HIPing),extrusion or other thermomechanical working. Such methods ofconsolidation are capable of providing suitable P/M sheet bars with aweight of up to several hundred pounds, e.g., five hundred pounds, onethousand pounds or more. Subsequent thermomechanical processing of theP/M sheet bar is applied similarly to then P/M derived refractory metalsas to metals from ingot sources.

The present invention inhibits grain growth in niobium P/M sheets duringhigh temperature exposure. A low oxygen niobium powder (<about 400 ppm,preferably <about 200 ppm) is needed as a starting material. Powderswith a higher content in oxygen cannot be consolidated to full densityand/or will not yield good mechanical properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing a process of the present invention tocreate stabilized grain size powder; and

FIGS. 2-4 are flow charts showing examples of consolidating steps tocreate products made of stabilized grain size powder.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention relates to a process for making a metal mill product froma refractory metal powder comprising (a) providing a low oxygenrefractory metal powder; (b) adding to the powder a grain growthinhibitor to the low oxygen refractory metal powder before consolidatingthe powder, (c) consolidating the powder by either hot isostaticpressing, extrusion or another thermomechanical working process; and (d)subjecting the consolidated powder to subsequent thermomechanicalprocessing, and thereby forming the mill product. The invention alsorelates to products made from such a process.

The low oxygen niobium powder can be any powder, which when used inaccordance to the invention, enables user to meet an object of theinvention. The metal powders with stabilized grain size of the presentinvention are preferably produced via the following procedure asdiscussed in U.S. Pat. No. 6,261,337, incorporated herein in itsentirety. Niobium alloys can also be used.

In other embodiments, instead of using niobium powders, powders madefrom a refractory metal selected from hafnium, molybdenum, niobium,rhenium, tantalum, tungsten, vanadium, and zirconium metals can be used.Also, alloys of these metals can also be used.

As illustrated in FIG. 1, low oxygen niobium and grain growth inhibitorpowders (for example SiO₂ or Y₂O₃) are blended to form low oxygen powderwith grain size inhibitors. FIGS. 2-4 illustrate the consolidation stepswith the master blend. The physical processes of blending andconsolidating achieve a uniform distribution of grain growth inhibitingparticles in the powder metal sheet bars. The powders are made by theprocess described in U.S. Pat. No. 6,261,337 and as described herein.

These powders are blended to produce the desired alloy composition. Thepowders are then sealed in an evacuated can, heated to a desiredtemperature, and extruded such that the extrusion ratio is at least 8:1.This is done to completely consolidate the niobium powders and theincluded inhibitors. The can may be removed either just before or justafter the rolling operation.

The above process can afford advantages of more stable grain size in thefinal material, more uniform material properties (such as ultimatetensile strength and hardness), lower manufacturing costs, bettercontrol of fiber size, and greater flexibility for alloy modificationsand control of properties.

Niobium sheets produced from powder blends of niobium and graininhibitors, for example silicon, were tested for grain growth, ultimatetensile strength, and hardness. The test results are presented in Table1 below. TABLE 1 1150° C. Ultimate 1065° C. @180 1300° C. TensileSilicon @90 min min @180 min Strength Hardness (ppm) (ASTM) (ASTM)(ASTM) (KSI) (VICKERS)  0 9.5 9.5 7.5 49.3 114 150 9.5 9.0 8.0 50.3 117300 9.5 9.5 8.5 49.5 125 Nb I/M 5.5 <1 <1 32 72

P/M sheet with grain growth inhibitors, preferably silicon, of 0, 150,and 300 ppm were thermomechanical processed to a thickness of 0.015inches and annealed at 1065° C. for 90 minutes to produce grain sizes ofapproximately ASTM 9.5. Niobium sheet produced from ingot metallurgy(I/M) a grain size of approximately ASTM 5.5 under the same anneal heattreat conditions. The P/M and I/M test samples were subjected toadditional annealing heat treatments at 1150° C. for 180 minutes and1300° C. for 180 minutes. The P/M test samples yielded grain sizesgreater than ASTM 7.0 compared to I/M test samples that yielded grainsizes coarser than ASTM 1.

Additionally, the higher P/M Ultimate Tensile Strength of 49.3 KSI, 50.3KSI, and 49.5 KSI and hardness of 114 VHN, 117 VHN, and 125 VHN aresignificant improvements over typical I/M material of Ultimate TensileStrength of 32 KSI and hardness of 72 VHN. The fine grain sizes andimproved tensile strength and hardness after heat treatment of the P/Mmaterial is a significant advantage, compared to I/M material, inapplications where large amounts of deformation are required duringfabrication, such as deep drawn diamond cups, or capacitor cans.

Alternatively, the blended powders may be isostatically pressed into abar prior to canning and extrusion, as illustrated in FIG. 2. Theadvantage of this method would be to put a higher weight into thecompact prior to extrusion to aid in consolidation and increase yieldper extrusion.

Now returning to FIG. 1, niobium hydride powder is placed into a vacuumchamber, which also contains a metal having a higher affinity foroxygen, such as calcium or magnesium, preferably the latter. Preferably,the starting hydride powder has oxygen content less than about 1000 ppm.The chamber is heated to the dehydration temperature to remove thehydrogen, then heated to the deoxidation temperature to produce a powderof niobium or alloy of niobium having a target reduced oxygen content ofless than about 400 ppm preferably below 200 ppm and more preferablybelow 100 ppm. The magnesium, containing the oxygen, is then removedfrom the metal powder by evaporation and subsequently by selectivechemical leaching or dissolution of the powder.

For example, a niobium powder with less than 400 ppm oxygen can beproduced by the deoxidization of niobium hydride under partial pressureof argon. Niobium hydride powder would be blended with 0.3 wt.-%magnesium and placed in a vacuum furnace retort, which is evacuated, andbackfilled with argon. The pressure in the furnace would be set at about100 microns with Argon flowing and the vacuum pump running.

The furnace temperature would be ramped to about 650° C. inapproximately 50° C. increments, held until temperature equalized, thenramped up to 950° C. in approximately 50° C. increments. When thetemperature equalized at 950° C. it would be held for about two hours.After such hold, the furnace is shut down. Once the furnace cools itspowder content is removed from the retort.

The magnesium, containing the oxygen, would then be removed from themetal powder by acid leaching to produce the resulting niobium powderhaving an oxygen content of less than 300 ppm.

As described above, in the process for producing formed powder metalproducts of niobium, the metal hydride powder is deoxidized to an oxygencontent of less than about 400 ppm. The powder is consolidated to form aniobium or alloy product, having an oxygen content below about about 400ppm, or below about 300 ppm or below about 200 ppm or below about 100ppm, but for many powder metallurgy purposes between about 100 ppm and150 ppm. According to the present invention, a formed refractory metalproduct (niobium product), having a stabilized grain size, may beproduced from metal hydride powder, as treated as described above, byany known powder metallurgy techniques.

Exemplary of these powder metallurgy techniques used for forming theproducts are the following, in which the steps are listed in order ofperformance. Any of the following single techniques or sequences oftechniques may be utilized in the present invention: cold isostaticpressing, sintering, encapsulating, hot isostatic pressing andthermomechanical processing; cold isostatic pressing, sintering, hotisostatic pressing thermomechanical processing; cold isostatic pressing,encapsulating, hot isostatic pressing and thermomechanical processing;cold isostatic pressing, encapsulating and hot isostatic pressing;encapsulating and hot isostatic pressing; cold isostatic pressing,sintering, encapsulating, extruding and thermomechanical processing;cold isostatic pressing, sintering, extruding, and thermomechanicalprocessing; cold isostatic pressing, sintering, and extruding; coldisostatic pressing, encapsulating, extruding and thermomechanicalprocessing; cold isostatic pressing, encapsulating and extruding;encapsulating and extruding; mechanical pressing, sintering andextruding; cold isostatic pressing, sintering, encapsulating, forgingand thermomechanical processing; cold isostatic pressing, encapsulating,forging and thermomechanical processing; cold isostatic pressing,encapsulating and forging; cold isostatic pressing, sintering, andforging; cold isostatic pressing, sintering and rolling; encapsulatingand forging; encapsulating and rolling cold isostatic pressing,sintering and thermomechanical processing; mechanical pressing andsintering; and mechanical pressing, sintering, repressing andresintering; other combinations of consolidating, heating and deformingmay also be utilized.

The production of a formed niobium product having a stabilized grainsize can be achieved by cold isostatic pressing of various kinds ofknown niobium powders to form a compact, followed by a hot isostaticpressing (HIPing) step to densify the compact and then thermomechanicalprocessing of the powder compact for further densification andcompletion of the bonding, as illustrated in FIG. 3. Preferably, niobiumpowder with grain size inhibitors would be cold isostatically pressed at60,000 pounds/sq. in. and room temperature, into a compact withrectangular or, preferably, round cross section, then hermeticallyencapsulated and hot isostatically pressed (HPed) at 40,000 lbs. 1 sq.in. and 1300° C. for four hours. The HIPed compact would beunencapsulated and converted to sheet or foil by thermomechanicalprocessing steps.

A similar process, as illustrated in FIG. 4, of just cold isostaticpressing, sintering and thermomechanical processing using niobium powderhaving an oxygen content of less than 300 ppm can be conducted by coldisostatically pressing at 60,000 lbs./sq. in. into a bar shape preform.This preform would be sintered at 1500° C. for two hours in a vacuum ofless than about 0.001 Torr to yield a preform having a density of about95% theoretical density (Th) and less than 400 ppm oxygen. The sinteredpreform would be converted into sheet and foil by thermomechanicalprocessing steps.

Production of a formed niobium sheet or foil having a stable grain sizeby hot extrusion and thermomechanical processing can be made, usingniobium powder having an oxygen content of less than 400 ppm as thestarting powder. This powder can be hermetically encapsulated thenextruded through a rectangular or, preferably, round die at 1000° C. toproduce an extruded product having oxygen content of less than 400 ppm.The extruded product can be converted to sheet or foil by thethermomechanical processing.

Niobium sheet or foil with oxygen content of less than 400 ppm can beproduced by cold isostatic pressing, hot extrusion and thermomechanicalprocessing. This compact made by cold isostatically pressing could behermetically encapsulated then extruded at 1000° C. to produce anextruded product with an oxygen content of about 300 ppm which can beconverted into sheet and foil by thermomechanical processing steps.

Niobium products having stable grain size can be prepared by mechanicalpressing, sintering, repressing and resintering.

Niobium powder blend having oxygen content of less than 400 ppm can beutilized as the starting powder. It is placed in a die and mechanicallypressed, using uniaxial pressure. The pressed tablet should be thensintered at 1500° C. for two hours in a vacuum evacuated to less thanabout 0.001 Torr. The sintered tablet would then be repressed andresintered at 1500° C. for two hours in a vacuum evacuated to less thanabout 0.001 Torr.

The resintered tablet will have oxygen content of less than about 400ppm and be suitable for thermomechanical processing to produce a formedniobium product.

In one embodiment, a copper or steel container is filled with niobiumpowder, evacuated, hermetically sealed, and extruded through a die togive a 10:1 extrusion ratio. The copper container is removed by acidtreatment and the extruded bar is thermo-mechanically processed into asheet form flat. In another embodiment, a steel container is filled withthe niobium powder, evacuated, hermetically sealed and HIPed. The steelcontainer is removed by machining and the HIPed piece is thermomechanically processed into a sheet form flat.

Anneals may be used to improve workability of the material in betweentwo deformation steps or to adjust grain size and texture throughrecrystallization although a final anneal may not be necessary. When thepowder is canned during the consolidation (usually to protect it fromthe environment at high temperature), the can will bond to the niobium.

In another embodiment, the process provides P/M sheets of large size(>100 pounds) having good mechanical properties and small stable grainsize, capable of a higher yield than conventional P/M processes forsheet manufacture, typically 50 pounds or less. Low oxygen niobiumpowder of less than 400 ppm, preferably less than 150 ppm, ofnon-spherical particles and sizing less than 250 microns FAPD (FisherAverage Particle Diameter), is provided per processes described herein.Powders with a higher content in oxygen cannot be consolidated to fulldensity and/or will not yield good mechanical properties. The powder isconsolidated to full density either by HIPing (hot isostatic pressing)or by extrusion. Both methods of consolidation are capable of providingsuitable P/M sheet bars with a weight of up to several hundred pounds.

Thermomechancial processing of the P/M sheet bar is similar to standardprocesses.

Numerous variations and modifications may obviously be made withoutdeparting from the present invention. Accordingly, it should be clearlyunderstood that the forms of the present invention herein described areillustrative only and are not intended to limit the scope of theinvention.

1. A process for making a metal mill product from a refractory metalpowder comprising: (a) providing a low oxygen refractory metal powder;(b) adding to the powder a grain growth inhibitor to the low oxygenrefractory metal powder before consolidating the powder; (c)consolidating the powder by either hot isostatic pressing; extrusion oranother thermomechanical working process; and (d) subjecting theconsolidated powder to subsequent thermomechanical processing, andthereby forming the mill product.
 2. The process of claim 1, wherein therefractory metal is niobium or a niobium alloy.
 3. The process of claim1, wherein the refractory metal is selected from the group consisting ofhafnium, molybdenum, rhenium, tantalum, tungsten, vanadium, andzirconium metals, alloys of the foregoing metals, and combinationsthereof.
 4. The process of claim 1, wherein prior to consolidating thepowder, the grain growth inhibitor is added to the powder by (i)blending an inhibitor component with the powder or (ii) a residue of ade-oxidation process.
 5. The process of claim 4, wherein the residue isa residue formed in a de-oxidation process, wherein magnesium is addedto capture the oxygen from the niobium powder and magnesium oxide formsduring the de-oxidation process.
 6. The process of claim 4, wherein theinhibitor component is selected from the group consisting of SiO₂, Y₂O₃,and mixtures thereof.
 7. The process of claim 1, wherein the low oxygenniobium powder has an oxygen content that is less than about 400 ppm. 8.The process of claim 1, wherein the low oxygen niobium powder has anoxygen content that is less than about 300 ppm.
 9. The process of claim1, wherein the low oxygen niobium powder has an oxygen content that isless than about 200 ppm.
 10. The process of claim 1, wherein the lowoxygen niobium powder has an oxygen content ranging from about 100 ppmto about 150 ppm.
 11. The process of claim 1, wherein the low oxygenniobium powder has an oxygen content that is less than about 100 ppm.12. The process of claim 1, wherein the mill product is a sheetcontaining oxide particles.
 13. The process of claim 1, wherein the millproduct is a foil.
 14. The process of claim 1, wherein the mill productis a sheet weighing at least 100 pounds.
 15. A mill product comprising astabilized grain size made from a process comprising: (a) providing alow oxygen refractory metal powder; (b) adding to the powder, beforeconsolidating the powder, a grain growth inhibitor to the low oxygenrefractory metal powder, (c) consolidating the powder by either hotisostatic pressing, extrusion or another thermomechanical workingprocess; and (d) subjecting the consolidated powder to subsequentthermomechanical processing, and thereby forming the mill product. 16.The process of claim 15, wherein the refractory metal is niobium or aniobium alloy.
 17. The mill product of claim 15, wherein the refractorymetal is selected from the group consisting of hafnium, molybdenum,rhenium, tantalum, tungsten, vanadium, and zirconium metals, alloys ofthe foregoing metals, and combinations thereof.
 18. The mill product ofclaim 15, wherein prior to consolidating the powder, the grain growthinhibitor is added to the powder by blending an inhibitor component or(ii) a residue of a de-oxidation process.
 19. The mill product of claim15, wherein the residue is a residue formed in a de-oxidation process,wherein magnesium is added to capture the oxygen from the niobium powderand magnesium oxide forms during the de-oxidation process.
 20. The millproduct of claim 18, wherein the inhibitor component is selected fromthe group consisting of SiO₂, Y₂O₃, and mixtures thereof.
 21. The millproduct of claim 15, wherein the low oxygen niobium powder has an oxygencontent that is less than about 400 ppm.
 22. The mill product of claim15, wherein the low oxygen niobium powder has an oxygen content that isless than about 300 ppm.
 23. The mill product of claim 15, wherein themill product is a sheet or a foil.
 24. A process for making a metal millproduct from a niobium powder comprising: (a) providing a low oxygenniobium powder having an oxygen content that is less than about 400 ppm;(b) adding to the powder a grain growth inhibitor to the low oxygenniobium powder before consolidating the powder by blending an inhibitorcomponent or (ii) a residue of a de-oxidation process, wherein theresidue is a residue formed in a de-oxidation process, wherein magnesiumis added to capture the oxygen from the niobium powder and magnesiumoxide forms during the de-oxidation process, (c) consolidating thepowder by either hot isostatic pressing, extrusion or anotherthermomechanical working process; and (d) subjecting the consolidatedpowder to subsequent thermomechanical processing, and thereby formingthe mill product.